I would like to use Oracle Grid Infrastructure to protect a single-instance database.
I found these docs:
http://www.oracle.com/technetwork/products/clusterware/overview/em-db-failover-128585.pdf (for 10.2)
http://www.oracle.com/technetwork/products/clusterware/overview/si-db-failover-11g-134623.pdf (for 11.1)

I was unable to find a similar document for GI 11.2 . Do you know if such a document exists?

At the moment I only know that the GI is free if used for an Oracle Product, but I am not shure whether it can be used to protext external applications or single-instance databases

while this still works (the scripts for failover you find in the demo folder $GI_HOME7/crs/demo/coldfailover), the document does not exists, since with 11gR2 there is a supported solution called RAC ONE Node, hence rendering the manual setup obsolete.